Source .cpp files on gist with Rcpp::sourceCpp (#511)

In https://github.com/wush978/devtools/commit/8470f0f790bb74a9e3f72cf55e98d1d0b370b7e4 to https://github.com/wush978/devtools/commit/93fe2304807a357aedb43978a75ad588842a8b35 :

- `souce_gist` will use either `source` or `Rcpp::sourceCpp` according to the file extension. The related parsers are implemented in `. parseJSON1`.
- Some tests are implemented in `tests/testthat/test-gist.r`
- All files in gist will be downloaded and renamed accordingly, so they could `source` each other with argument: `chdir = TRUE` with relative path. Please check the second test in `tests/testthat/test-gist.r`
- Some documentations are modified.